Orion 9877 manual Equatorial mounting

The equatorial mounting...

Setting up and using an

equatorial mount

he Earth's eastward rotation causes Tstars to apparently rotate around a point in the sky which is a projection of the Earth's axis, called the celestial pole. A star's path through the sky from an earthbound observer's viewpoint is therefore an arc of this rotation; it rises in the east towards its highest altitude (on the local meridian - directly south) and then sets in the west. It does not travel in a line parallel to the horizon. (Except at the north pole, from where the celestial pole is at

the zenith!)

Telescopes are generaly provided with one of two basic designs of mountings. The so called alt-az allows a telescope to move in altitude (up/down) and azimuth (left/right). This does not allow easy tracking of a star's curved path through the sky since an alt-az has to move in a series of vertical and horizontal steps, which for the purpose of steady observation, are cumber- some.

In the Equatorial, the alt-az mounting is in effect tipped at an angle, provision being made for the azimuth axis to be aligned to the Earth's rotational axis (towards the celestial pole). This allows a tele- scope to follow a star by simply countering the Earth's rotation in one motion, rather than the combination of two movements which would be necessary with an alt-az. A motive force, either manual or motorised, applied to what we must now call the 'polar' axis, is all that is necessary to track a star through the sky.

In order for this action to follow a star without deviation the polar axis of the equatorial mounting

must be very accurately aligned on the celestial pole. The better this is done, the less requirement there will be for minor adjustments during observation.

Polar axis alignment

(approximate setting)

Try to carry out polar axis alignment on a level surface, where the location of the tripod (or pillar) feet can be permanently recorded to allow the telescope to be placed in exactly the same position for subsequent observations. If a choice of observing location is limited, try to position the telescope for the best view of the southern sky as possible.

Adjust the angle on the latitude scale of the polar axis, shown in figure 1, until this equates to your latitude (previously deter- mined from an OS map).

For the sake of ease and safety, it is recom- mend that the telescope and its counter- weights be removed from the mounting for this exercise.

The polar axis itself should then be aligned north-south, with its upper end facing true north. (Not magnetic north!) An approximate heading can be obtained by sighting along the polar axis so that it points towards Polaris.

With such approximate alignment, the tele- scope should be able to track objects with only occasional corrections on the declination axis.

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